Conventionally, as for a superconducting material, metal based or oxide based material is known. Typical metal based superconducting materials are niobium titanium (NbTi), niobium-3 tin (Nb3Sn), niobium-3 germanium (Nb3Ge), or the like. However, among the metal based superconducting materials, even for niobium-3 germanium (Nb3Ge), the critical temperature is 23 K (Kelvin), thereby, there is a problem that expensive liquid helium have to be used for cooling.
As for an oxide based superconducting material, since lanthanum (La) oxide based superconducting material of critical temperature 30 K has been discovered, copper oxide based materials having critical temperature above the boiling temperature of nitrogen (77 K) are discovered in succession. The typical copper oxide based superconducting materials are yttrium (Y), bismuth (Bi), thallium (Tl), and mercury (Hg) based material, and the like. However, since the copper oxide based superconducting material is so-called ceramics, there is a problem that, because of its poor workability, it is difficult to make it a wire having excellent homogeneity in long-wire. The copper oxide based superconducting material also has large magnetic anisotropy, thereby it is required to align between the crystal orientations of a substrate or the sheath material and the superconducting phase. The work for aligning crystal orientations is very difficult, thereby, there is also a problem in yield or cost.
In 21st century, it has been discovered that MgB2 reveals superconductivity at 39 K. Since MgB2 has quite small magnetic field anisotropy, it is possible to attain high critical current density even if its crystal orientation is not aligned to the same orientation of the sheath material as the copper oxide based superconducting material.
Moreover, MgB2 has a critical temperature as higher as 20 K or more than that of the metal based conductive material. It is also reported that the upper critical magnetic field of MgB2 is about 40 T in a thin film. If these properties are utilized, it will be realistic for application under cooling of a refrigerator, or in a strong magnetic field.
It has been recognized that MgB2 has practically very effective features in that it can be made into a wire by a powder-in-tube method, and moreover, it is possible to attain practical critical current density even by mechanical processing only (See, for example; JP-A-2002-373534, abstract and paragraph [0028]). Thus, it means that high superconducting property can be attained without any heat processing. The feature is absolutely different from the conventional superconducting wire, in which superconductivity phenomena does not appear without heat processing. Since shortening of a manufacturing process, enlarging the degree of the selectivity of metal sheath material, and improving the degree of freedom for coil winding and designing are achieved by utilizing the features, it is considered that significant cost reduction can be achieved as compared to the conventional superconducting material.